The present invention relates to a process for producing superfines of metal. More particularly, the invention relates to a process for producing ferromagnetic particles with a single magnetic domain by a vapor-phase reaction.
With the recent increase in the demand for high-density magnetic recording mediums, magnetic particles are required to have improved characteristics, namely, high coercivity and great saturation magnetization. The second factor is material dependent, but the first factor has a maximum value when the individual particles of the material have a single magnetic domain and are in either an acicular or straight-chain form. Therefore, the ideal magnetic material can be provided from superfines of metal having a single magnetic domain.
The structure of the magnetic domain depends on the particle size of a magnetic material. For large particles, a structure with more than one magnetic domain is predominant, but as their size is decreased, a structure with a single magnetic domain becomes predominant, and with an even smaller particle size, super-paramagnetism comes into action. While the particle size that provides a single magnetic domain varies with the type of metal or alloy, iron and cobalt particles have a single magnetic domain at a size in the range of 10 to 30 nm.
Superfines of a magnetic metal are known to be made of metallic iron particles or alloy particles wherein iron is alloyed with vanadium, chromium, manganese, cobalt, nickel, copper or zinc. These superfines of metal are typically produced by either oxide reduction or vapor condensation. In the first method, acicular iron oxide or oxyhydroxide particles prepared by a suitable technique such as wet precipitation are reduced into superfines of pure iron by heating in hydrogen atmosphere at low temperatures ranging from 300.degree. to 400.degree. C. The resulting particles are in most cases acicular and their size is 50 nm by 300-700 nm. However, these particles tend to have internal voids, and magnetization that occurs in these voids provides a structure having more than one magnetic pole which is detrimental to the uniform dispersion of magnetic particles in a magnetic paint and which hence impairs the orientation in magnetic tape or reduces its coercivity. As a further disadvantage, in order to prevent sintering during reduction, the fine oxide particles must be heated at low temperatures for an extended period, and this requires large equipment and leads to great consumption of hydrogen.
The second method or vapor condensation involves forming the vapor of iron or iron-cobalt alloy in an argon gas in a low degree of vacuum. This method provides superfines of metal of a size of 5-50 nm in the form of long chains. However, this method requires the use of an expensive heating furnace and evacuating chamber, and the working under vacuum is uneconomical because of low efficiency and productivity. Other problems with the use of vacuum are low cooling rate and the increased chance of sintering of deposited particles. The junction of single particles is easily sintered so as to provide a structure having more than one magnetic domain. Fine particles with this structure are either in the form of curved chains or a net of intertwined agglomerates.
One of the inventors of the present invention has filed Japanese Patent Application No. 127415/80 (and corresponding U.S. Pat. No. 4,383,852) wherein he proposed a process for producing fine metal particles by a vapor-phase reaction consisting of reacting a reducing gas with the vapor of a metal halide having a lower boiling point than a metal. This method provided fine particles of iron-copper, iron-nickel or iron-nickel-cobalt alloy having a size of 40-600 nm. However, much finer particles of a size of 10-30 nm having a structure with a single magnetic domain were difficult to obtain by this method.